1. Field of the Invention
The present invention relates to a hot air burner/generator with internal ignition.
It is notably but not exclusively applied to gas burners with an intrinsically cold nozzle capable of carrying out an external combustion of a high velocity gas mixture.
2. Description of the Prior Art
It is known that burners of this type are frequently applied to low temperature heating of plastic film, for example polyethylene film in view of their retraction, by means of a gas flow from the combustion of a combustible gas such as propane and air.
In order to satisfy this type of application, the burner should therefore be designed so as to produce a gas flow having a temperature of the order of 120 to 540° C. at a predetermined distance from the burner (a distance at which the plastic film to be treated should be positioned).
At this distance, the temperature should be relatively homogenous and the gas flow free of combusting material, if the intention is to avoid any possibility of burning, singeing and blistering of the film.
In order to achieve this result, a burner has already been proposed, comprising an injection device capable of producing a flow of a combustible gas mixture at a high velocity and injecting this flow into a burner head with a tubular shape successively including:
a pressure recovery chamber having, in the plane of symmetry of the head, a divergent shape and inside of which the gas mixture from the injection device develops according to a fan-shaped configuration,
an ignition chamber with a substantially constant and rectangular section,
two baffles which respectively extend along both large sides of the ignition chamber, and which converge towards each other, each baffle having two rectilinear front borders forming together a passage with reduced width, and
diffusion means which may comprise a grid or even a set of two grids, with a substantially hemicylindrical shape attached inside the head at the junction of both chambers, these diffusion means forming a bulging partition, with an axis parallel to said borders, in said plane of symmetry, and the concavity of which is oriented towards the inside of the pressure recovery chamber.
In this burner, the diffusion means may be made from wire netting or from a perforated metal sheet.
It is found that, by means of the structure described earlier, the burner head is not licked by the flame and therefore does not undergo any significant heating.
In order to homogenize the forefront of the flame and to avoid having inopportune orientations of the burner induce heterogeneities of the flame, the use of circular shapes for the front borders of the ignition chamber and the baffles as well as for the diffusion grid, was proposed (Patent Application FR 87 06930).
Moreover, it is known that in order to carry out ignition of the flame, the use of an ignition spark plug mounted in a tubular housing opening out into the ignition chamber by means of an orifice provided in a location of the wall of said chamber located at right angles to the side area of the grid, has already been proposed.
However, during use, it is reported that this solution has a serious drawback. Indeed, the tubular housing of the plug forms a cavity generating a turbulent state of gas flow emitted by the perforations of the grid. Consequently, self-sustained permanent combustion of the gas flow may be established at the level of this cavity, causing heating of the wall of the ignition chamber, which is contrary to the sought purpose.
In order to attempt to suppress this drawback, an ignition device was also proposed, involving an ignition cavity outside the ignition chamber and communicating with the ignition chamber by means of a through-orifice with a reduced section provided in the wall of the ignition chamber substantially at right angles to the front region of the grid, the section of this orifice being provided sufficiently small so that the perturbations of the gas mixture flow at this orifice cannot generate a self-sustained parasitic combustion hearth capable of heating the wall of the ignition chamber.
Nevertheless, this solution has a drawback resulting from the fact that during ignition, the generated spark causes sudden combustion of the gas mixture in the ignition chamber. The combusting gases are ejected and will then ignite the flow of gas mixture which flows out of the central perforations of the grid of the burner. This is a noisy solution which is unpleasant for the operator and persons who are present around it.
Moreover, it is found that a significant problem which has to be solved in the design of a direct ignition device is that of the voltage and of the energy of the electric pulses which should be applied to the ignition electrodes, in order to obtain efficient ignition of the gas mixture which flows at a high velocity inside the burner. Indeed, in order to achieve direct ignition, the gap between the electrodes should be relatively large. This voltage and this instantaneous energy should therefore be relatively high so that it is suitable to use a sufficiently powerful generator on the one hand, and an electric connection between the generator and the electrodes on the other hand, which is particularly well insulated so as to prevent line losses or perturbations.